Frequency control



Feb. 23, 1943. M. G. CROSBY 2,312,079

FREQUENCY CONTROL Filed Sept. 6, 1940 2 Sheets-Sheet 1 8 :9 H- m; Q0 E v% Il|| 64 1 T .1 J E FREQUENCY K299929999! .U7d1fl0 05103150 1 FRE UENCX INVENTOR MURRA Y ZROSBY BY ATTORNEY Feb. 23, 19437 M. G. CROSBY FREQUENCY CONTROL Filed Sept. 6, 1940 2 Sheets-Sheet 2 v INVENTOR M URRA V G. CROSBY ATTO R N EY Patented Feb. 23, 1943 FREQUENCY common Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation 01' America, a corporation of Delaware Application September 6, 1940, Serial No. 355,592

7 Claims.

This application discloses a frequency modulator which is stabilized as to its mean frequency by means of an automatic frequency control system which uses a piezo crystal as the frequency discriminator.

Frequency modulators of the type involved here have been disclosed in my United States application #136,578, filed April 13, 1937, Patent No. 2,279,659, April 14, 1942, and in my United States application #312,446, filed January 5, 1940, Patent No. 2,279,660, April 14, 1942. These prior systems utilize the ordinary tuned circuit type of discriminator which requires the use of heterodyning to a lower frequency to increase the sensitivity of the AFC (automatic frequency control) system so as to obtain a high degree of stability. In the system of this disclosure, a highly selective piezo crystal discriminator is used which obviates the necessity of heterodyning to a lower frequency, to obtain selectivity, and thereby simplifies the circuit. With this crystal discriminator, the mean frequency is stabilized to the wellknown degree that may be realized with crystal control of ordinary oscillator circuits.

In describing my invention reference will be made to the attached drawings, wherein:

Figs. 1 and 2 each show different embodiments of my new frequency stabilized frequency modulator; while Figs. 3a and 3b are curves illustrating the operation of the circuits.

In Fig. l, which will now be referred to, I show a specific embodiment of my stabilized modulator. Reactance tube l and oscillator 29 are of known types and comprise an oscillating system whose frequency may be controlled by varying an electrode voltage, say the voltage of grid !4, of the reactance tube ID. A simple triode oscillator comprises tube 20 having its grid 22 and anode 24 regeneratively coupled. A point on inductance 30 of oscillator tank circult 39-32 is coupled to the cathode 34. The plate 8 of the reactance tube I0 is coupled to a point on the oscillator circuit 3032, and by condenser C and resistance R to the cathode E2 of tube H). The grid 14 of tube I0 is coupled to a point between C and R so that the desired phase relation between the high frequency voltages on the plate and grid of tube In to produce the desired reactive effect in tube In is obtained. The reactive plate circuit of tube In is placed across a portion of the oscillating circuit including inductance 30, because the cathodes of tubes l0 and 20 are coupled together as are the anodes. Phase shifter C, R is adjusted so that C has a high reactance compared to the magnitude of resistor R. This causes a leading current to flow through R so that the current flowing in the plate circuit of I0 is produced by a leading voltage on the grid, and is therefore capacitive.

Modulating potentials are applied from transformer l8 through bypass condenser l9 to vary the voltage of grid I 4 of the reactance tube I0 and' thereby vary the frequency of the oscillator in the same accordance. Frequency modulated output energy is derived from the oscillator tube or circuits.

The AFC potentials are obtained from cathode resistors 40 and 42 which feed their differential voltage through a time-constant filter comprising resistance 45 and condenser 46. The timeconstant filter allows only the slow variations of the mean frequency of the frequency modulated oscillator output energy to pass a voltage to lead I1, and thus removes the detected modulation potentials. The crystal discriminator consisting of input electrodes 50, 52, crystal 54, and output electrodes 56, 58, feeds the plates of diodes 58 and 60. Chokes 6B and 58 furnish a direct-current return path for the diode rectiflers 5B and 50. Link circuit Ill couples oscillator energy from tank circuit 39, 32 to discriminator input circuit 64.

The crystal discriminator comprises a crystal in a four-electrode type of crystal holder. This type of holder has an output between one output electrode and ground whose characteristic is represented by the solid line curve of Fig. 3a, and an output between the other output electrode and ground whose characteristic may be represented by the dotted curve of Fig. 3a. When these two outputs are fed to the differential diode detectors 58 and 60, the resulting direct-current potential appearing across the diode resistors 40 and 42 varies with respect to the mean, or center, frequency of the applied frequency modulated energy as is shown in Fig. 3b. Thus, in the case of Fig. 32), when the frequency drifts high a positive voltage is fed to control lead I7, and when it drifts low a negative voltage is fed to lead ll. These control voltages act on the grid l4 of reactance tube In to change its reactance, and thus return the frequency generated by tube 20 towards the condition of zero detected output voltage which is the proper carrier frequency.

The crystal discriminator produces the characteristics of Fig. 30 by virtue of the capacity couplings which take place in the four-electrode crystal holder. Without capacity couplings in the holder, the characteristics of the two output circuits of the holder would be simple resonance characteristics without the rejection dips I and I2 shown in Fig. 3a. However, since there is natural capacity between electrodes 60 and 66, and between 50 and 50, the resulting characteristics have the shape of overand under-neutralized crystal filter characteristics. Since electrodes 56 and 58 are of opposite potential, the capacity between 50 and 56 tends to under-neutralize the characteristic appearing at 66, and the capacity between 50 and 58 tends to produce the opposite caracteristic, which is that of over-new, tralization.

In many cases the inherent capacity couplings in the crystal holder are suflicient to convert the characteristics from simple resonance curves to overor under-neutralized crystal filter characteristics. However, where additional control is desired, variable capacities CI and C2 may be connected as shown to supplement the crystal holder capacities. This gives a somewhat more accurate and flexible control of the characteristics obtained, since the variation of CI and C2 controls the placing of the rejection dip with respect to the carrier frequency which latter frequency is designated as FC in Fig. 3a.

One might think that due to the presence of frequency modulation of the oscillations the wave would be spread to an extent such that it would be broader than the crystal discriminator circuit and the same would not respond properly. However, I have found that when the master oscillator is tuned to a low frequency which must be multiplied many times, the degree of modulation present on the master oscillator is not too great for the crystal filter. For instance, suppose it were desired to radiate 40 megacycles with a 75 kilocycle frequency deviation. If the master oscillator were, for example 1.0 megacycle. the depth of modulation present on the master oscillator would be 40 of 75 or 1.875 kilocycles. My experience has been that the crystal filter will maintain control with much larger deviations than this 1.875 kilocycles of the example.

The embodiment of Fig. 2 makes use of the three-electrode type of crystal discriminator which has been disclosed in my United States application #195,041, filed March. 10, 1938, now Patent No. 2,204,575, dated June 18, 1940. This discriminator is also described in my article entitled Communication by phase modulation, published in the February 1939 issue of the Proceedings of the I. R. E.

This crystal filter comprises a crystal I06 in a holder having three electrodes I05, I01 and I 08. Of these electrodes I is coupled to a tuned input circuit comprising a condenser I03 and an inductance I04 coupled to link circuit I0. The output electrode I0! is coupled to the grid 2 of a coupling tube I I4. The output electrode I08 is coupled to the grid I I6 of coupling tube H8.

.of Fig. 1 in that an inductive type ofv reactance tube I0 is used, and in that the coupling tubes I I4 and H8 are used to drive the differential diodes I28 and I34. The 'reactance tube, consisting of tube I0, blocking condenser BC, and phase shifter R, C, is of the inductive type because the phase shifter R, C, feeds a lagging voltage to the grid I4 so that the plate circuit of tube I0 acts like a variable inductance circuit of oscillator 20. The phase shifter is adjusted so that R is large in comparison to the reactance of C' so that the current is in phase with the voltage applied to the phase shifter, that is, the voltage passed by condenser BC. The drop across C lags the current so that the tube acts as an amplified inductance which decreases in inductance as the amplification of the tube is increased.

The discriminator of Fig. 2 is fed by midtapped coil I04 which provides potential on the electrode I05 for driving the crystal andpotential of opposite polarity, by way of reactances I09 and III, to the crystal electrodes I01 and I00, respectively, for neutralizing the capacities between the crystal holder electrodes. Crystal I06 is held in a holder with one input electrode I05 and two output electrodes I01 and I08. I have found that these output electrodes act independent of each other, and may be separately neutralized by means of neutralizing condensers I08 and III. However, inorder to produce the proper characteristics for the discriminator, one of the outputs is under-neutralized and the other over-neutralized so as to produce output characteristics corresponding to the solid and dotted lines of Fig. 3a. In this discriminator the location of the rejection dips I0 and I2 is determined by the degree of off-neutralization which is controllable by the neutralizing condensers I09 and iii.

Tubes H4 and H8 are coupling and amplifier tubes which amplify the output energy of the crystal, and thereby allow it to work at a lower level so as to reduce heating and the possibility of fracture. If desired, transformers I24 and I32 may be replaced by tuned impedance coupling for feeding the diodes I28 and I34.

Other types of crystal discriminators which may be used to produce the required characteristic of Fig. 3a are disclosed in my United States application #186,273, filed January 22, 1938, Patent No. 2,204,574, June 18, 1940; United States application #242,469, filed November 26, 1938, now Patent #2,192,684, issued March 5, 1940; United States application #178,655, filed December 8, 1937, now Patent #2,156,376, issued May 2, 1939; United States application #167,344, filed October 5, 1937, now Patent #2,156,375, issued May 2, 1939; United States application #165,056, filed September 22, 1937, now Patent #2,156,374, issued May 2, 1939; and United States application #138,117, filed April 21, 1937, now Patent #2,172,732, issued September 12, 1939.

What is claimed is:

1. In a system for stabilizing the means frequency of frequency modulated oscillations flowing in an oscillation circuit, and which system is of the type employing a reactance tube having ananode, a cathode and an auxiliary electrode with a reactive circuit coupling its anode, auxiliary electrode and cathode, means coupling the electrodes of said tube to the oscillation circuit; the improvement which comprises a rectifier having its output circuit coupled to an electrode of said reactance tube, a discriminator filter including a single piezo-electric crystal, capacitive means imparting an over-neutralized characteristic and an under-neutralized characteristic to the filter.

' said filter being provided with output terminals coupled to said rectifier, and an oscillation transmission coupling between input terminals of said filter and said oscillation circuit. I

2. In a system for detecting variations in the frequency of wave energy from a selected mean frequency, a piezo-electric crystal in a holder having a plurality of electrodes, an alternating current circuit coupled to one of said electrodes for impressing wave energy the frequency of which may vary on said one electrode of said crystal holder, separate circuits coupled each with another electrode of said holder for deriving modified wave energy from said other electrodes of said crystal holder, a reactive coupling between said one electrode and one of said other electrodes of a value to over-neutralize the circuit in which said other electrode is coupled, a reactive coupling between said one electrode and the remaining one of said other electrodes of a value to under-neutralize the circuit in which said remaining electrode is coupled, and a rectifying system coupled to said separate circuits.

3. In a system for detecting variations in the frequency of wave energy from a selected mean frequency, a piezo-electric crystal in a holder having three electrodes, an alternating current circuit coupled to one of said electrodes for impressing wave energy the frequency of which may vary on said one electrode of said crystal holder, separate circuits coupled each with one of the remaining electrodes of said holder for deriving modified wave energy from said other electrodes of said crystal holder, areactive coupling between said one electrode and one of said remaining two electrodes of a value to over-neutralize the circuit in which said last mentioned electrode is coupled, a reactive coupling between said one electrode and the remaining one of said two other electrodes of a value to under-neutralize the circuit in which said last mentioned electrode is coupled, a rectifier coupled to each of said separate circuits and a. common output for said rectifiers.

4. In a system for detecting variations in the frequency of wave energy from a selected mean frequency, a piezo-electric crystal in aholder having two pairs of electrodes, an alternating current circuit coupled to one of said pairs oi electrodes for impressing wave energy the frequency of which may vary on said one pair of electrodes of said crystal holder,'separate circuits coupled each with an electrode of the remaining pair of electrodes of said crystal holder for deriving modified wave energy from said other pair of electrodes of said crystal holder,

a reactance coupling each electrode ofsaid last mentioned pair of electrodes to one electrode of said first mentioned pair of electrodes, one of said reactances being of a value to over-neutralize one of said separate circuits, the other of said reactances being of a value to under-neutralize the other. of said separate circuits, and a pair of rectifiers having a common output circuit coupled to said separate circuits.

5. In a wave frequency variation detector, a detectona wave filter network having a single piezo-electric crystal provided with an input circuit excited by wave energy of varying frequency, said filter network having an output circuit, ca-' pacitative means in said filter network foroifneutralizing the same with respect to the mean frequency of said wave energy, and means coupling the output circuit of said filter network to said detector.

6. In a wave frequency variation detector, a full wave rectifier having an input circuit and having an output circuit, a wave filter network having an input circuit and also having an output circuit which is coupled to the input circuit of said full wave rectifier, capacity means'in said filter network for over-neutralizing the same with respect to the mean frequency of wave energy of changing frequency to be detected, a

second wave'filter network having an input circuit and also having an output circuit coupled to the input circuit of said full wave rectifier, capacity means in said second wave filter network for under-neutralizing the same with respect to the mean frequency of said wave energy 'of V changing frequency to be detected, a single piezoelectric crystal common to said two filter networks and means-for impressing said wave energy of changing frequency on the input circuit of said wave filter networks. a

7. In combination with a source of frequency modulated energy whose frequency deviations are substantial relative to the mean frequency, a

discriminator-rectifier network comprising a pair of differentially connected rectifiers having a common output load, a discriminator input netcoupling said input circuit to said crystal, means coupling the crystal to said rectifiers, and means providing for said filter overand under-neutralized characteris tics with respect to said mean frequency.

MURRAY G. CROSBY. 

